A Re - Assessment of the Criteria for Certifiability in CCRMP

The effect of the between- and within-laboratory standard deviation and of the number of laboratories on the magnitude of the certification factor, CF, introduced in 1975 by the Canadian Certified Reference Materials Project (CCRMP) has been discussed. The applicability of CF to reference materials has been assessed on the basis of the correlation between the between- and within-laboratory standard deviation. The validity of the critical value of 4 for CF for certifiability is examined.
An examination of all available data has led to the conclusion that the quality of an interlaboratory program is sufficient to warrant certifiability only if the ratio of the between-to the within-laboratory standard deviation is 3 or less for most elements and 2 or less for uranium. It has been shown that the number of laboratories rejected based on the upper limit of this ratio differs only slightly from that based on laboratory means differing by more than twice the standard deviation of the overall mean for the majority of elements that have been certified. A new measure for assessing the certifiability of an element has been introduced and is based on the requirement that the percentage of laboratories that are rejected to give a ratio of the between- to within-laboratory standard deviation less than or equal to the critical value of 3 or, for uranium, 2 should be no greater than 15% regardless of the number of laboratories in the certification program.